Corey Stewart was Virginia state chairman for Donald Trump in 2016.
Photograph: AP

A Republican candidate to challenge former vice-presidential nominee Tim Kaine for a Senate seat in Virginia has suggested Barack Obama’s birth certificate is a forgery, in a tweet that paired the debunked conspiracy theory with the controversial Alabama Senate race.

Stewart’s reference to the “AllredYearbookFraud” was a response to reports on Friday that one of the women who has accused GOP Alabama Senate candidate Roy Moore of sexual misconduct when she was a teenager added text to what she says is Moore’s signature in her high-school yearbook.

Gloria Allred is the celebrity attorney who is representing the accuser, Beverly Nelson. Moore, who is endorsed by Trump, has denied all accusations against him.

On Sunday, Stewart announced that he would help Moore get out the vote in the last two days of campaigning.

Trump – who also faces and denies multiple claims of sexual misconduct – has long been a leading voice in the so-called “birther” movement, which is built on the claim that Obama was not born a US citizen and so was not qualified to be US president.

Trump used the movement to aid his transition from reality TV to politics. In 2011, while flirting with a run against Obama, he claimed to have investigators in Hawaii researching the incumbent’s birth and said: “They cannot believe what they’re finding.”

In April 2011, Obama had Hawaii release his full long-form birth certificate. Trump accepted the document and conceded during his 2016 campaign that Obama was born in the US. The New York Times reported last month, however, that Trump still privately questions his predecessor’s legitimacy.

On Saturday, a representative for Stewart did not immediately respond to a request for comment.

Stewart is a self-styled “conservative Republican” who was fired as Trump’s Virginia chair in October 2016, after he staged a rally in front of Republican National Committee headquarters, seeking to warn the party not to replace Trump as candidate.

He narrowly lost the Republican primary for the 2017 governor’s race, in which Gillespie lost to the Democrat, Ralph Northam, by a punishing nine-point margin.

The RNC’s backing for the controversial Moore in Alabama notwithstanding, it has been reported that party bodies are looking to block Stewart’s Senate run in Virginia, a state long slipping from red to blue which produced a tide of Democratic victories this year.